Code review feedback.

* Make `FunctionInfo` public and `CompiledModule::func_info` return it.
* Make the `StackMapLookup` trait unsafe.
* Add comments for the purpose of `EngineHostFuncs`.
* Rework ownership model of shared signatures: `SignatureCollection` in
  conjunction with `SignatureRegistry` is now used so that the `Engine`,
  `Store`, and `Module` don't need to worry about unregistering shared
  signatures.
* Implement `Func::param_arity` and `Func::result_arity` in terms of
  `Func::ty`.
* Make looking up a trampoline with the module registry more efficient by doing
  a binary search on the function's starting PC value for the owning module and
  then looking up the trampoline with only that module.
* Remove reference to the shared signatures from `GlobalRegisteredModule`.

crates/wasmtime/src/trampoline/func.rs

@@ -270,7 +270,11 @@ pub fn create_function(
     // If there is no store, use the default signature index which is
     // guaranteed to trap if there is ever an indirect call on the function (should not happen)
     let shared_signature_id = store
-        .map(|s| s.register_signature(ft.as_wasm_func_type(), trampoline))
+        .map(|s| {
+            s.signatures()
+                .borrow_mut()
+                .register(ft.as_wasm_func_type(), trampoline)
+        })
         .unwrap_or(VMSharedSignatureIndex::default());
 
     unsafe {